Localized electrostatic wave packets in the frequency region of lower hybrid waves have been detected by the instruments on the FREJA satellite. These waves are often associated with local density depletions indicating that the structures can be interpreted as wave filled cavities. The basic features of the observations are discussed. On the basis of simple statistical arguments it is attempted to present some characteristics which have to be accommodated within an ultimate theory describing the observed wave phenomena. An interpretation in terms of collapse of nonlinear lower hybrid waves is discussed in particular. It is argued that such a model seems inapplicable, at least in its simplest form, by providing a timescale and a length scale which are not in agreement with observations. Alternatives to this model are presented.1.

Field and particle data recorded on the geostationary satellite GEOS 2 are used to investigate the electric and magnetic signatures of a substorm characterized by a dispersionless injection of energetic electrons and ions. Three types of field variations are observed: (1) Long-period oscillations with period of • 300 s, interpreted as oscillations of entire field lines. These oscillations develop as second harmonic standing waves and correspond to coupled shear Alfv•n-slow magnetosonic modes. They grow after the most active period of the breakup. (2) Short-period transient osci•ations with periods of • 45-65 s, interpreted as wave modes trapped in a current layer which develops prior to the substorm breakup and is disrupted at breakup. These osci•ations also correspond to a coupled shear Alfv•n-slow magnetosonic mode (coupled via magnetic field curvature effects in a high-fi plasma). The short-period transient osci•ations are only observed during the most active period of the breakup. (3) A nonoscillatory sharp increase observed on both the parallel magnetic component and the energetic ion flux, averaged over one satellite rotation, interpreted as evidence for the fast magnetosonic mode which in view of the simultaneous large impulsive increase in the azimutha• electric field, appears to propagate radially outwards, transporting the substorm breakup downtail. Paper number 95JA00990. 0148-022 7 / 95/95J A-00990505.00 Alfv•n oscillations associated with this substorm, the electric and magnetic field measurements have been reanalyzed with new data analysis techniques. A characterization of the low-frequency oscillations at and after substorm breakup is in fact one of the prerequisites for understanding the overall mechanisms of substorm onset and recovery. We are faced with the following questions: Do the oscillations trigger the breakup, or are they a consequence of the breakup? In somewhat different terms, do they appear at onset or at a later time? Do they play a role in generating the auroral westward traveling surge (WTS) and the associated discrete auroral forms (DAF)? To answer such questions, what must be known is the frequency spectrum of the oscillations, as well as their polarizations and modal structures, the ti. me of onset, and the maximum amplitude of each frequency component, subject to the limitations of the uncertainty in the simultaneous localization of frequency and time of each transient. This is no easy task, for a number of reasons. The eigenfrequencies of the oscillation s at the satellite location are not constant in time, since the shape, length, and mass loading of the corresponding field line vary rapidly during the dipolarization process associated with the substorm; hence the determination of a time-varying eigenmode frequency spectrum is required. Furthermore, the duration of the physically significant wave activity associated with the substorm may last no more than 15 to 20 min, whereas the wave eigenperiods may be of the order of 10 min or more. Such low-frequency oscillations, if de...

Abstract. Two time periods, each covering both quiet and disturbed conditions (growth phase, breakup, and postbreakup phase), are studied. Electric and magnetic field measurements, carried out in the near-Earth plasma sheet (NEPS), are used to calculate the two components (radial and azimuthal) of the electric E x BIB • drift.These calculations are compared with independent estimates of the ion flow direction deduced from ion flux measurements. During active periods, the two flow directions coincide to a large degree. Evidence is given for two regimes of transport: (1) During the growth phase, and after the active phase, the electric field (radial and azimuthal) and hence the azimuthal and radial flow velocities are small in the near-equatorial region. This is interpreted as the consequence of an electrostatic field that tends to shield the induced electric field associated with time-varying external conditions. (2) During active phases (breakup and pseudobreakup), however, large-amplitude bursts in E x B/B e radial and azimuthal components (interpreted as flow bursts), with typical velocities of the order of 100 km s -1, are observed. The direction of these flow bursts is somewhat arbitrary, and in particular, for the two substorm events described here, sudden reversals in the flow direction are observed. These fast flow bursts coincide with intense low-frequency electromagnetic fluctuations: current-driven Alfv•n waves (CDA waves) with frequency f _• f•r+, the proton gyrofrequency. CDA waves produce "anomalous" collisions on timescales shorter than the electron bounce period, thus violating the second adiabatic invariant for electrons. As a consequence, the electrostatic shielding is destroyed, which leads to enhanced radial transport. Thus the transport in the NEPS seems to be controlled by a microscopic current-driven instability.

Abstract. We study the development of substorm breakups characterized by dispersionless injections of energetic particles at the geostationary orbit. The corresponding magnetic signature is a fast change from tail-like to dipolelike configuration with transient superimposed low frequency oscillations (T~I mn). We show that intense waves (fib --1 nT) with shorter periods (1 s) systematically develop at breakup, and that their intensification is strongly related to the dipolarization and to the fast increase of energetic electrons. These "higher frequency" (F ~ 1 Hz) waves appear as short lasting bursts, strongly confined across the magnetic field. Hence they look like kinetic Alfv6n waves and are likely to have finite parallel electric fields, thereby resonating with electrons. We compute the diffusion coefficient and show that electrons are heated along the parallel direction and can gain up to 5 keV in a few tens of seconds. This fast parallel diffusion of electrons leads to cancellation of the parallel current and therefore to a complete modification of the current system.

The nonlinear development of the E × B-instability in a weakly ionized plasma is governed by an infinite set of coupled ordinary nonlinear differential equations for the amplitudes of the modes of the system. The solution of these equations are investigated in the adiabatic approximation by setting the time derivative of linearly stable modes equal to zero. Approximate analytical solutions for the time development of the different modes, in good agreement with previous numerical results, have been obtained for control parameters fifteen times the critical value. The results clearly indicate the possible existence of a cascading process, where the amplitudes of the linearly most stable modes have the highest nonlinear growth rates.